Is the NACA Scoop really functional?

Ever wonder about that cool-looking air intake on the hood ? Did you wonder if it were actually functional? Considering that the Viper is a no-nonsense beast to begin with, you probably have assumed it's not just there for looks alone. But you may be surprised to learn when it actually starts working.

Being an aircraft design engineer, I'm exposed to NACA scoops on a weekly basis. A 747 has five of them (almost as many as a Ferrari F40 :-). Anyway, I wanted to do a quick, simplified analysis of the Viper's NACA scoop, and see how it performs as you go faster.

First, let me explain something called dynamic pressure. This is basically a pressure increase due to going fast. Normal atmospheric pressure is about 14.7 pounds per square inch ( psi ) at sea level. But if you put a scoop in front of the air stream as you go 600 mph at sea level, you'll have not only 14.7 psi of air pressure, but an additional 6.5 psi of effective "boost"! Fifty percent more air pressure means 50% more power.

Don't confuse the air pressure with the volume of air flowing into the scoop. The cubic feet of air per minute (CFM) trying to ram into the scoop depends only the size of the scoop. Working pressure and flow rate aren't the same thing.

To illustrate this dymanic pressure thing, check out the graph below. You can think of it as how much "free" boost you get depending on how fast you're travelling. Sadly, this is not a straight-line relationship. You have to go 225 mph in order to get one measly psi of boost.

A NACA scoop is not the most efficient scoop, but it's a good trade-off between integrating it into the curve of the hood and, importantly, minimizing drag. Only about 70% of the air trying to ram into it is succeeding. But it looks a heck of a lot nicer than a big blower scoop sticking out of the hood.

I made a simplified graph of how much CFM of air the engine needs as the Viper accelerates at wide-open throttle. This is assuming a stock 3.07 rear gear, by the way. You can see where the shifts occur. On the same graph, I plotted the estimated performance of the NACA scoop, based on its geometry. And I did factor in the dynamic pressure benefit, as well. You might notice that this line isn't perfectly straight as a result.

Below speeds of 120 mph, the NACA scoop does help minimize pumping losses (the act of sucking air into the motor)) by supplying the engine with a source of moving air, but you can see that you're not really gaining anything significant below this speed, because the engine still requires more air than the NACA scoop is helping to provide. However, once you get in fourth gear around 120 mph, the NACA scoop officially becomes functional, and by the time your, um, Venom 600 hits 200 mph, that NACA scoop is trying its best to shove in 40% more air than the motor needs at an effective boost pressure of 0.7 psi. Too bad it's not providing 40% more air pressure, though. But I'd put money on the fact that that NACA scoop is responsible for an extra 40+ hp at that speed.

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